"""Heterograph NN modules"""
import tensorflow as tf
from tensorflow.keras import layers

__all__ = ['HeteroGraphConv']

class HeteroGraphConv(layers.Layer):
    r"""A generic module for computing convolution on heterogeneous graphs.

    The heterograph convolution applies sub-modules on their associating
    relation graphs, which reads the features from source nodes and writes the
    updated ones to destination nodes. If multiple relations have the same
    destination node types, their results are aggregated by the specified method.
    If the relation graph has no edge, the corresponding module will not be called.

    Pseudo-code:

    .. code::

        outputs = {nty : [] for nty in g.dsttypes}
        # Apply sub-modules on their associating relation graphs in parallel
        for relation in g.canonical_etypes:
            stype, etype, dtype = relation
            dstdata = relation_submodule(g[relation], ...)
            outputs[dtype].append(dstdata)

        # Aggregate the results for each destination node type
        rsts = {}
        for ntype, ntype_outputs in outputs.items():
            if len(ntype_outputs) != 0:
                rsts[ntype] = aggregate(ntype_outputs)
        return rsts

    Examples
    --------

    Create a heterograph with three types of relations and nodes.

    >>> import dgl
    >>> g = dgl.heterograph({
    ...     ('user', 'follows', 'user') : edges1,
    ...     ('user', 'plays', 'game') : edges2,
    ...     ('store', 'sells', 'game')  : edges3})

    Create a ``HeteroGraphConv`` that applies different convolution modules to
    different relations. Note that the modules for ``'follows'`` and ``'plays'``
    do not share weights.

    >>> import dgl.nn.pytorch as dglnn
    >>> conv = dglnn.HeteroGraphConv({
    ...     'follows' : dglnn.GraphConv(...),
    ...     'plays' : dglnn.GraphConv(...),
    ...     'sells' : dglnn.SAGEConv(...)},
    ...     aggregate='sum')

    Call forward with some ``'user'`` features. This computes new features for both
    ``'user'`` and ``'game'`` nodes.

    >>> import tensorflow as tf
    >>> h1 = {'user' : tf.random.normal((g.number_of_nodes('user'), 5))}
    >>> h2 = conv(g, h1)
    >>> print(h2.keys())
    dict_keys(['user', 'game'])

    Call forward with both ``'user'`` and ``'store'`` features. Because both the
    ``'plays'`` and ``'sells'`` relations will update the ``'game'`` features,
    their results are aggregated by the specified method (i.e., summation here).

    >>> f1 = {'user' : ..., 'store' : ...}
    >>> f2 = conv(g, f1)
    >>> print(f2.keys())
    dict_keys(['user', 'game'])

    Call forward with some ``'store'`` features. This only computes new features
    for ``'game'`` nodes.

    >>> g1 = {'store' : ...}
    >>> g2 = conv(g, g1)
    >>> print(g2.keys())
    dict_keys(['game'])

    Call forward with a pair of inputs is allowed and each submodule will also
    be invoked with a pair of inputs.

    >>> x_src = {'user' : ..., 'store' : ...}
    >>> x_dst = {'user' : ..., 'game' : ...}
    >>> y_dst = conv(g, (x_src, x_dst))
    >>> print(y_dst.keys())
    dict_keys(['user', 'game'])

    Parameters
    ----------
    mods : dict[str, nn.Module]
        Modules associated with every edge types. The forward function of each
        module must have a `DGLHeteroGraph` object as the first argument, and
        its second argument is either a tensor object representing the node
        features or a pair of tensor object representing the source and destination
        node features.
    aggregate : str, callable, optional
        Method for aggregating node features generated by different relations.
        Allowed string values are 'sum', 'max', 'min', 'mean', 'stack'.
        The 'stack' aggregation is performed along the second dimension, whose order
        is deterministic.
        User can also customize the aggregator by providing a callable instance.
        For example, aggregation by summation is equivalent to the follows:

        .. code::

            def my_agg_func(tensors, dsttype):
                # tensors: is a list of tensors to aggregate
                # dsttype: string name of the destination node type for which the
                #          aggregation is performed
                stacked = tf.stack(tensors, axis=0)
                return tf.reduce_sum(stacked, axis=0)

    Attributes
    ----------
    mods : dict[str, nn.Module]
        Modules associated with every edge types.
    """
    def __init__(self, mods, aggregate='sum'):
        super(HeteroGraphConv, self).__init__()
        self.mods = mods
        # Do not break if graph has 0-in-degree nodes.
        # Because there is no general rule to add self-loop for heterograph.
        for _, v in self.mods.items():
            set_allow_zero_in_degree_fn = getattr(v, 'set_allow_zero_in_degree', None)
            if callable(set_allow_zero_in_degree_fn):
                set_allow_zero_in_degree_fn(True)
        if isinstance(aggregate, str):
            self.agg_fn = get_aggregate_fn(aggregate)
        else:
            self.agg_fn = aggregate

    def call(self, g, inputs, mod_args=None, mod_kwargs=None):
        """Forward computation

        Invoke the forward function with each module and aggregate their results.

        Parameters
        ----------
        g : DGLHeteroGraph
            Graph data.
        inputs : dict[str, Tensor] or pair of dict[str, Tensor]
            Input node features.
        mod_args : dict[str, tuple[any]], optional
            Extra positional arguments for the sub-modules.
        mod_kwargs : dict[str, dict[str, any]], optional
            Extra key-word arguments for the sub-modules.

        Returns
        -------
        dict[str, Tensor]
            Output representations for every types of nodes.
        """
        if mod_args is None:
            mod_args = {}
        if mod_kwargs is None:
            mod_kwargs = {}
        outputs = {nty : [] for nty in g.dsttypes}
        if isinstance(inputs, tuple):
            src_inputs, dst_inputs = inputs
            for stype, etype, dtype in g.canonical_etypes:
                rel_graph = g[stype, etype, dtype]
                if rel_graph.number_of_edges() == 0:
                    continue
                if stype not in src_inputs or dtype not in dst_inputs:
                    continue
                dstdata = self.mods[etype](
                    rel_graph,
                    (src_inputs[stype], dst_inputs[dtype]),
                    *mod_args.get(etype, ()),
                    **mod_kwargs.get(etype, {}))
                outputs[dtype].append(dstdata)
        else:
            for stype, etype, dtype in g.canonical_etypes:
                rel_graph = g[stype, etype, dtype]
                if rel_graph.number_of_edges() == 0:
                    continue
                if stype not in inputs:
                    continue
                dstdata = self.mods[etype](
                    rel_graph,
                    (inputs[stype], inputs[dtype]),
                    *mod_args.get(etype, ()),
                    **mod_kwargs.get(etype, {}))
                outputs[dtype].append(dstdata)
        rsts = {}
        for nty, alist in outputs.items():
            if len(alist) != 0:
                rsts[nty] = self.agg_fn(alist, nty)
        return rsts

def get_aggregate_fn(agg):
    """Internal function to get the aggregation function for node data
    generated from different relations.

    Parameters
    ----------
    agg : str
        Method for aggregating node features generated by different relations.
        Allowed values are 'sum', 'max', 'min', 'mean', 'stack'.

    Returns
    -------
    callable
        Aggregator function that takes a list of tensors to aggregate
        and returns one aggregated tensor.
    """
    if agg == 'sum':
        fn = tf.reduce_sum
    elif agg == 'max':
        fn = tf.reduce_max
    elif agg == 'min':
        fn = tf.reduce_min
    elif agg == 'mean':
        fn = tf.reduce_mean
    elif agg == 'stack':
        fn = None  # will not be called
    else:
        raise DGLError('Invalid cross type aggregator. Must be one of '
                       '"sum", "max", "min", "mean" or "stack". But got "%s"' % agg)
    if agg == 'stack':
        def stack_agg(inputs, dsttype):  # pylint: disable=unused-argument
            if len(inputs) == 0:
                return None
            return tf.stack(inputs, axis=1)
        return stack_agg
    else:
        def aggfn(inputs, dsttype):  # pylint: disable=unused-argument
            if len(inputs) == 0:
                return None
            stacked = tf.stack(inputs, axis=0)
            return fn(stacked, axis=0)
        return aggfn
